A common problem associated with the heterogeneous polymerization of monomers (for example vinyl chloride or other vinylidene comonomers) is the deposition of polymer on the interior surfaces of the polymerization vessel. These surfaces include the stainless steel walls of the vessel, the blades of the agitator and other metal surfaces in contact with the monomer or monomers. This scale buildup will decrease the yield of polymer product, the cooling capacity of the equipment, as well as the production efficiency of the reactor. Moreover, if the scale comes off the reactor surfaces and admixes with final product (to cause so-called "fish eyes"), the quality of the product may be substantially and adversely affected.
Due to these severe problems, it is necessary to remove any scale deposited following each polymerization run. Various kinds of scale removal methods have been proposed and employed such as solvent cleaning, hydraulic and mechanical reactor cleaners, and the like. However, this cleaning operation not only requires extensive time and labor but also causes health hazards to the workers in view, for example, of the carcinogenic property possessed by vinyl chloride monomer. Efforts have also been made to develop additives for the polymerization reaction to prevent the polymer scale on the inner walls of the reactor.
Recent developments have focused on protective coatings on the inside wall of the vessel. The surfaces of the reactor coming into contact with the monomer or monomers are coated prior to the polymerization run with dyes, or polymeric material. For example, U.S. Pat. Nos. 4,320,215; 4,076,951; 4,007,320; 4,200,712 and 4,080,173 are directed to this approach. In this regard, condensation polymers derived from polyhydric phenols and aldehydes have been found to be effective in some areas for the commercial production of PVC resins but still have some disadvantages. A good coating must have easy application procedures, stability with respect to the polymerization medium and good adhesion to the walls of the reactor.